What is Rinkeby (Ethereum Testnet)?

Rinkeby is Ethereum's testnet and is built for testing smart contracts, network architecture, and other experimental Ethereum technology before it goes live on its mainchain. Rinkeby employs a Proof-of-Authority (PoA) consensus mechanism that allows only a small number of nodes to mine blocks on the network as opposed to Ethereum Execution Layer's (formerly Ethereum 1.0) Proof-of-Work (PoW) architecture that allows any node with enough computational power to participate.